Method of making a prestressed reinforced concrete structure



METHOD OF MAKING A PRESTRESSED REINFORCED CONCRETE STRUCTURE Filed Oct.15, 1957 A. R. SCHWAB July 3, 1962 2 Sheets-Sheet 1 lNl/ENTGR AL VIN E..SCHWAB Attorney United States Patent 3,041,702 METHOD OF MAKING APRESTRESSED REINFORCED CONCRETE STRUCTURE Alvin R. Schwab, Glen Rock,N.J., assignor to United States Steel Corporation, a corporation of NewJersey Filed Oct. 15, 1957, Ser. No. 690,295

6 Claims. (Cl. 25-154) This invention relates to a method of making aprestressed reinforced concrete structure, particularly beams or otherstructures in which the reinforcing steel, for best etficie'ncy, shouldbe bent. There are two methods of prestressing concrete, namely,pretensioning and post-tensioning. In pretensioning high strength wire,wire strand or rod is stretched between two fixed points or abutments.While the steel is maintained in its elongated condition, concrete ispoured around it and after the concrete has attained sufficientstrength, the ends of the steel are released. The stress in the steel isthen transferred to the concrete by means of the bond between the twomaterials. In post-tensioning, the concrete is poured before the steelis stretched with bond between the two materials being prevented.Ordinarily, the stress in the steel is transferred to the concrete bymeans other than bond. The result produced is essentially the same asthat produced by the pretensioning method. The pretensioning method hasmany advantages over post-tensioning, but pretensioning has adisadvantage that, in the method ordinarily used, the steel is mostconveniently stretched in a straight line from abutment to abutment. Formaximum economy, the prestressing force at the center of the span of abeam should be applied as closely as possible to the bottom of the beam,While to eliminate tension in the top fibers it must ordinarily beplaced somewhat closer to the centroid at the ends of the beam. However,if the steel is to lie in a straight line, it is impossible to place thesteel in the position described. There have been various attempts toovercome this difficulty in pretensioning concrete. The shape of themember has been varied toward the ends (as, for example, by haunching ordeepening the beam) and the amount of pretensioned force has beendecreased by destroying or preventing bond between some portion of thesteel and the concrete near the beam end, but these expedients onlypartially solve the problem. Methods have also been devised to raise thesteel at the ends of the beam or depress it near the center of the beam,but this is difficult on long prestressing beds or with heavy beamsrequiring a large amount of bent-up steel.

One difficulty in bending up the steel is that large vertical forcesoccur at the point where the steel is bent upwardly. For example, whenstressing a group of ten diameter steel strands to 14,000 lbs. perstrand and bending up through an angle of lo degrees, there will be avertical component of 25,000 lbs. so that there is a problem in holdingdown the prestressing strand. Either massive weights or prestressingbeds with great beam strength may be required.

It is therefore an object of my invention to provide a method of makinga prestressed reinforcing concrete structure by pretensioning in whichthe reinforcing steel is bent up without applying any appreciablevertical forces to the prestressing bed.

This and other objects will be more apparent after referring to thefollowing specification and attached drawings. in which:

FIGURE 1 is a schematic sectional elevation of a prestressing bedshowing intermediate steps of my method;

FIGURE 2 is a view, similar to FIGURE 1, showing a second embodiment ofmy invention;

FIGURE 3 is an enlarged view of a detail of FIGURES 1 and 2;

3,041,702 Patented July 3, 1962 FIGURE 4 is a fragmentary view, showinga modification of the detail of FIGURE 3;

FIGURE 5 is a transverse sectional view of the detail of FIGURE 4;

FIGURE 6 is a transverse sectional view of a further embodiment of myinvention;

FIGURE 7 is a schematic elevation showing still another embodiment of myinvention;

FIGURE 8 is a view of a detail of my invention;

FIGURE 9 is a view, similar to FIGURE 7, showing a modification thereof;6

FIGURE 10 is a view, similar to FIGURE 7, showing a modificationthereof;

FIGURE 11 is a view, similar to FIGURE 7, showing a modificationthereof;

FIGURE 12 is a view of a detail of a part of FIGURES 10 and 11; and

FIGURE 13 is a view of another detail of a part of FIGURES 10 and 11.

Referring more particularly to the drawings, the reference numeral 2indicates a prestressing bed having abutments 4 at the ends thereof. Aneyebolt 6 passes through each of the abutments 4 and has nuts 8 and 10thereon, one on each side of the abutment 4. Two tension members 12 and14 have their ends fastened to the eyebolts 6. The tension members 12and 14 may be of any of the types commonly used in making prestressedconcrete. In most instances the members 12 and 14 will be of greaterlength than the distance between eyebolts 6. Two or more struts orcompression members 16 are inserted between the tension members 12 and14 intermediate their ends and hold the tension members in spaced apartrelationship. The necessary tensioning force is then applied to themembers 12 and 14 in any suitable conventional manner. As shown, thismay be done by loosening one or both of the nuts 10 and tightening oneor both of the nuts 8. Another common method is to utilize a jack tomove one or both of the eyebolts 6 outwardly. Concrete is then pouredinto form 18 around the tension member 12. After the concrete sets therequired amount the tension members 12 and 14 are released from theeyebolts 6 and that portion of the tension member 12 extending beyondthe form may be cut on. The member 14, which acts as a dummy member, maybe reused. After suitable curing the beam B is ready for use. In makingthe beam disclosed in FIGURE 2 the procedure is the same as thatdescribed above except that the dummy tension member 14 is onlypartially outside the beam B1 with a recess 20 being provided at eachend of beam B1 for receiving the member 14. If desired recess 20 may beomitted and tension member 14 covered in any suitable manner to preventbonding of the member 14 to the concrete. In the embodiments of FIGURES1 and 2 a third tension member 22 may be provided adjacent the bottom ofthe prestressing bed 2. This member 22 is horizontal and is fastened tothe abutments 4 and tension applied thereto in the same manner as to themembers 12 and 14. The strut or compression member 16 may assumedifferent forms. As shown in FIGURE 3 a saddle 24 rests on the tensionmember 12 and the strut in the form of a bar 26 extends upwardly througha tube 28 to a second saddle 24 which receives the tension member 14.The tube 28 may be made of metal, cardboard, plastic or the like andmerely functions to prevent the concrete from bonding with the member 26After the prestress has been transferred to the concrete the members 14and 26 are removed and the hole in the tube 28 filled with concrete. Insome cases it is desirable to use a tie wire 30 from the strut 16 or 26to each of the members 12 and 14, thus preventing the strut from movinglongitudinally of the prestressing bed 2.

In the embodiment shown in FIGURES 4 and S the members 12 are bentaround a tube 32 which passes tr-ansversely from wall to wall of theform 18. A pin 34 passes through the tube 32 and projects beyond thetube on both sides of the form. The strut in the form of two bars 36,one bearing on each end of the pin 34, extends upwardly outside the form18 to a second pin 34 around which the members 14 pass. A tie wire 37extending from tube 32 around member 12 may be provided. After theconcrete is poured and the prestress transferred to the concrete themembers 34 and 36 are removed and the tube 32 filled with concrete.

In FIGURE 6 side forms 38 for the beam are utilized as the strut orcompression member. A tube 40 extending between the sides of form 38holds down the prestressed strands 12. A pin 42 passes through the tube40 and extends outwardly at each end of form 38. A second pin 44 restson top of the form 38 and holds up the dummy strands 14. Flangestiffeners 46 may be fastened to the form 38 to stiffen the sameadjacent the pins 42 and 44. After prestress has been transferred to theconcrete the pins 42 and 44 are removed along with the dummy strands -14and reused and the hole in the tube 40 filled with concrete.

FIGURE 7 shows an embodiment of my invention in which a plurality ofprestressed concrete structures may be formed at one time. In thisembodiment a prestressing bed 48 has a plurality of forms 50 thereinspaced apart end to end. A tension member 52 extends between and isfastened to end abutments 54 in the same manner as in FIGURE 1 by meansof eyebolts. A second tension member 56 is arranged above the member 52and is also fastened to abutments 54 by means of eyebolts arranged abovethe first set of eyebolts. Compression members 58', which may be of thesame types described above, are arranged to separate the tension members52 and 56 within the forms 50. A tension collar 60 holds the members 52and 56 together between each pair of forms 50. The tension collar mayassume Various forms but is shown in FIGURE 8 as made of two bent barsheld together with bolts. The prestressing is applied in the same manneras described above and, after the concrete has set the desired amount,dummy tension member 56 is removed along with the struts 58 and tensioncollars 60. Tension member 52 is separated between the forms 50 so as toform a plurality of beams B.

In the embodiment shown in FIGURE 9 three prestressing tension members62, 64 and 66 are fastened to abutments 54, one above the other in thesame manner as that described above. A dummy tension member 68 isfastened to the abutments 54 above the member 66. Forms 50 are providedin the same manner as described above. A strut 70 is provided betweenthe members 62 and 64 adjacent each end of each form 50 and a strut 72is provided between members 64 and 66 at each end of each form 50 buttoward the center of the form from struts 70. A strut 74 is providedbetween members 66 and 68 inwardly from each of the struts 72. A tensionring 76 is provided between each pair of forms 50 to hold the members 62and 68 together. This method of making the beams requires less dummysteel because the amount of external vertical force necessary to keepthe steel in a deflected position is reduced. The upward force from themember 62 at strut 70 and the downward force from the member 64 at thestrut 70 balance each other and in a similar manner the upward forcefrom the member 64 and the downward force from the member 66 at thestrut 72 balance each other. The downward force in the member 62 and theupward force in the member 66 at the strut 74 are carried by the member68.

FIGURES and 1-1 show a method of making pointed piles or othernon-prismatic forms with symmetrical placed prestressed steel. In thisembodiment, forms 78 are placed in prestressing bed 48 and tensionmembers 80 are arranged in a circle and attached to abutments 54 byeyebolts 6 in the same manner as described above. A compression ring 82is provided in each of the forms to change the direction of travel ofthe members 80. The compression ring 82 has a plurality of notches 84around its periphery for receiving the members 80. As shown, four forms78 are provided and a tension ring 86 is provided between each end formand the form adjacent thereto. The tension ring 86 has an opening 88therethrough with notches 90 therearound for receiving the members 80.When assembled tension is applied to the members 80 in the same manneras described above, concrete is poured and after the prestressing hasbeen transferred to the concrete, strands 80 are cut oflf at the end ofthe formed concrete.

In FIGURE 11 a plurality of forms 92 are arranged in the prestressingbed 48 and a plurality of tension members 94 are attached to theabutments 54 by eyebolts 6 in the same manner as in FIGURE 1. Acompression ring 96, similar to ring 82, is provided between each endform 92 and each abutment and also between the middle forms. A tensionring 98, similar to tension ring 86, receives tthe strands 94 betweeneach end form 92 and the form adjacent thereto. The tension is appliedto the members 94 and the piles P formed in the same manner as describedwhen discussing FIGURE 10.

While several embodiments of my invention have been shown and describedit will be apparent that other adaptations and modifications may be madewithout departing from the scope of the following claims.

I claim:

1. The method of making a prestressed reinforced concrete structurecomprising providing a prestressing bed having an abutment at each end,positioning two tension members in said bed with the ends supported atsaid abutments, inserting a compression member between said tensionmembers intermediate their ends to bend them out of alignment, applyingthe necessary pretensioning force to said tension members, then pouringconcrete around only one of said tension members to form -a concretestructure, and after the concrete has at least partially set removingthe other of said tension members.

2. The method of making a prestressed reinforced concrete structurecomprising providing a prestressing bed having an abutment at each end,positioning two tension members in said bed with their ends adjacenteach other at said abutments, inserting a compression member betweensaid tension members intermediate their ends to bend them out ofalignment, applying the necessary pretensioning force to said tensionmembers, then pouring concrete around only one of said tension membersto form said concrete structure, and after the concrete has at leastpartially set removing the other of said tension members.

3. The method of making a prestressed reinforced concrete structureaccording to claim 2 including the steps of positioning a third tensionmember in said bed with its ends spaced from the ends of the first andsecond tension members, and applying the necessary pretensioning forceto said third tension member, said concrete being poured around saidthird tension member before removing the first tension member.

4. The method of making a prestressed reinforced concrete structurecomprising providing a prestressing bed having an abutment ateach end,positioning two tension members in said bed with their ends adjacenteach other at said abutments, inserting two spaced apart compressionmembers between said tension members intermediate their ends, to bendthem out of alignment, applying the necessary pretensioning force tosaid tension members, then pouring concrete around only one of saidtension members to form said concrete structure, and after the concretehas at least partially set removing the other of said tension members.

5. The method of making a plurality of prestressed reinforced concretestructures comprising providing a prestressing bed having an abutment ateach end, positioning) two tension members in said bed with their endssupported at said abutments, placing a plurality of forms in said bed,inserting a compression member between said tension members intermediatetheir ends to hold them apart and bend them out of alignment, placing atension ring around said tension members to hold them togetherintermediate the compression member and an abutment, applying thenecessary pretensioning force to said tension members, then pouringconcrete around only one of said tension members to form said concretestructure, and after the concrete has at least partially set removingthe other of said tension members.

6. The method of making a plurality of prestressed reinforced concretestructures comprising providing a prestressing bed having an abutment ateach end, positioning a plurality of tension members in said bed withtheir ends supported at said abutments, placing a plurality of forms insaid bed in spaced relationship, inserting a compression member betweensaid tension members intermediate their ends within said forms to holdthem apart and bend them out of alignment with at least one tensionmember being above said form, placing a tension ring around said tensionmembers to hold them together between said forms, applying the necessarypretensioning force to said tension members, then pouring concrete intosaid forms around the tension member therein, and after the concrete hasat least partially set removing the tension member above said form.

References Cited in the file of this patent UNITED STATES PATENTSMcCarthy July 7, 1891 McCarthy Sept. 29, 1891 Burkholder et al. June 17,1905 Owens et a1. May 7, 1907 Wilson Mar. 19, 1918 Schorer Dec. 1, 1942Schorer Aug. 31, 1943 Barger et al. June 13, 1950 Blaton May 12, 1953Lipski Dec. 6, 1955 FOREIGN PATENTS Canada Aug. 12, 1952 Germany Sept.15, 1952 Great Britain Apr. 1, 1889 Great Britain June 10, 1920 GreatBritain Apr. 21, 1954 Great Britain Oct. 2, 1957 Italy July 25, 1950OTHER REFERENCES Equpiment (Magazine), 1957, pp. 138-141 and p.

